This invention generally relates to thermal insulating devices and methods and more particularly relates to a device and method for thermally insulating a structure to prevent thermal shock therein, which structure may be a feedwater inlet nozzle of the kind typically found on nuclear steam generators.
Although thermal insulating devices and methods are known, it has been observed that such devices and methods have a number of operational problems associated with them which make these devices and methods unsuitable for thermally insulating nuclear steam generator feedwater inlet nozzles to prevent thermal shock therein. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a typical nuclear steam generator and its associated feedwater inlet nozzle.
In this regard, a typical nuclear steam generator, such as is associated with pressurized water nuclear reactors, generates steam when heat is transferred from a heated and radioactive primary fluid to a non-radioactive secondary fluid (i.e., feedwater) of lower temperature. The secondary fluid flows into the steam generator through a feedwater inlet nozzle attached to the steam generator. The inlet nozzle is in fluid communication with a perforated feedring disposed in the steam generator. As the secondary fluid flows into the feedring, it also flows through the perforations of the feedring. The heated primary fluid flows through a plurality of tubes disposed in the steam generator as the secondary fluid simultaneously flows through the feedwater nozzle and the perforations of the feedring to surround the exterior surfaces of the tubes. The walls of the tubes conduct heat from the heated primary fluid flowing through the tubes to the secondary fluid of lower temperature surrounding the exterior surfaces of the tubes. As heat is transferred from the primary fluid to the secondary fluid, a portion of the secondary fluid vaporizes into steam which is piped to a turbine-generator for generating electricity in a manner well known in the art.
However, the temperature of the feedwater inlet nozzle may be substantially higher than the temperature of the relatively cold secondary fluid or feedwater flowing into the steam generator through the feedwater inlet nozzle. This temperature difference may be as great as approximately 100 degrees Fahrenheit during normal operation or 500 degrees Fahrenheit during transient conditions and may subject the nozzle to a phenomenon commonly referred to in the art as "thermal shock".
With respect to such transient conditions, relatively cold (e.g., 32 degrees Fahrenheit) secondary fluid from the auxiliary feedwater system is delivered to the feedwater nozzle during certain transient conditions. Such inflow of cold feedwater can cause thermal cycling and can induce the previously mentioned "thermal shock" in the nozzle. "Thermal shock" is defined herein as mechanical or thermal stress induced in a material due to rapid temperature changes in the material. Such "thermal shock" may induce metal fatigue in the nozzle. Such metal fatigue may in turn decrease the useful life of the nozzle and the steam generator to which it is attached. Therefore, a problem in the art is to mitigate the effects of "thermal shock" that may be experienced by the feedwater inlet nozzle in order to reduce metal fatigue therein so that the useful design life of the steam generator is not decreased. Maintaining the useful life of the steam generator avoids the cost of replacing the steam generator, which replacement cost may be approximately $30 million dollars. It is therefore desirable to mitigate the effects of "thermal shock" in the feedwater inlet nozzle in order to avoid the costs associated with replacing the steam generator.
More specifically, relatively cold (e.g., 32 degrees Fahrenheit) from the auxiliary feedwater system is delivered to the feedwater nozzle during certain transient conditions. Such inflow of cold feedwater is commonly referred to as thermal cycling and can induce the previously mentioned "thermal shock" in the nozzle.
Thermal insulating devices and methods are known. A device for reducing circumferential thermal gradients along the length of a feedwater inlet nozzle is disclosed in U.S. Pat. No. 4,057,033 issued Nov. 8, 1977 in the name of John Schlichting titled "Industrial Technique."According to this patent, an inlet feedwater nozzle is provided with a nozzle shroud to eliminate circumferential thermal gradient buildup in the nozzle at low flow rates and is also provided with a thermal sleeve-flange juncture to protect the nozzle from the thermal stresses resulting from large feedwater temperature changes. However, the nozzle of the Schlichting patent is not connected to a feedring and therefore is apparently unusable in steam generators of current design.
Hence, although thermal insulating devices and methods are known in the prior art, the prior art does not appear to disclose a device and method for suitably insulating a structure to prevent thermal shock therein, which structure may be a nuclear steam generator feedwater inlet nozzle.
Therefore, what is needed is a device and method for thermally insulating a structure to prevent thermal shock therein, which structure may be a feedwater inlet nozzle of the kind typically found on nuclear steam generators.